The invention relates to a magnetron cathode for a rotating tubular target, wherein the plasma is in the shape of a racetrack with two long straight stretches joined together.
In a known magnetron sputtering apparatus, a target is provided as a coating on the outer cylindrical surface of a copper support tube which is mounted for rotation in a vacuum chamber. Inner and outer loops of magnets are arranged inside the support tube to form a closed tunnel of magnetic flux which serves to trap a plasma loop over the target.
FIGS. 1 and 2 illustrate such apparatus, which is also applicable for the present invention. The target 1 is applied to support tube 2 having closed ends which form a drum. Axles in the form of tubes 5, 6 are journaled for rotation in walls 3, 4 of the vacuum chamber and serve as conduits for coolant 11 whose flow is indicated by arrows 7 and 8. The drum is sealed against leakage by seals 13 and 14. A row of magnets 17 on a yoke 19 and holder 21 inside the drum serves to concentrate a plasma 12 outside the target when power is supplied to the cathode.
In FIG. 2 the magnets 17 are part of the inner loop and have a single polarity facing radially outward. The magnets 18 are part of the outer loop and have the opposite polarity facing radially outward. A racetrack shaped yoke 19 on a holder 21 serves to complete the flux path.
FIG. 2A is a schematic section of another known rotating cathode system having a single central row of magnets 16 and an outer loop 17 on a yoke 20. Once again a racetrack shaped plasma loop is entrapped by the closed loop of magnetic flux 10. A magnetron of this type is disclosed in U.S. Pat. No. 5,047,131.
FIG. 3 illustrates the racetrack shape of the plasma having straight stretches 22, 23 which parallel the axis of rotation, and end stretches or turns 24, 25. A point 26 on the rotating target 44 moves in the direction of arrow 27 through the end stretch 25, while a point 28 moves in the parallel direction 29 through the side stretches 22, 23. Since the point 26 is exposed to the active area of the plasma longer than the point 28, the target 44 forms an ablation profile 35 as shown (exaggerated) in FIG. 4. If the ring-like pits 30, 31 eroded in the surface of the target 44 reach the support tube 2 (FIG. 1), the coating of the substrate will be contaminated. If the support tube 45 is eroded through, cooling water can be released into the vacuum chamber and cause major damage.
For additional discussion of rotatable cylindrical magnetrons, see Wright et al., "Design advances and applications of the rotatable cylindrical magnetron", Journal of Vacuum Science Technology A4(3), May/June 1986, pp. 388-392.